1. Field of the Invention
The present invention relates to an electrically programmable semiconductor memory device, and particularly relates to a semiconductor memory device in which a program control operation is performed.
2. Description of the Related Art
As shown in FIG. 11, a related semiconductor memory device includes a block decoder 10, voltage supply circuit CG drivers 6, a VPP pump circuit 30, a VPASS pump circuit 40, and a VRDEC driver 150. The VRDEC driver 150 shown in FIG. 12 has NMOS D-type transistors 152 connected in series, and a voltage VPGMH is inputted to gates of the transistors 152 via a level shifter 151. A voltage VPGMH is inputted to their one end, and a control signal VRDEC is outputted from the other end side. Moreover, NMOS D-type transistors 154 connected in series are provided between the control signal VRDEC and a voltage VDD, and a control signal VRDEC_V is inputted to gates of the transistors 154 via an inverter 153.
A memory region includes, for example, NAND-type flash memory cells. In FIG. 11, a top-bottom direction is a row direction, and a left-right direction is a column direction. The number of memory cells in the row and column directions is set properly according to memory capacity.
FIG. 13 shows the voltages of some nodes in this semiconductor memory device as program operation waveforms. At a time t0, a block is selected by an inputted address, and the VRDEC driver 150 outputs the voltage VPGMH to the row decoder in response to the control signal VRDEC_V. Namely, when the signal VRDEC_V goes to the voltage VDD at the time t0, an output of the control signal VRDEC starts to rise from the voltage VDD to the voltage VPGMH, that is, 20 V+Vtn. Program data is outputted to bit lines BL0 to BLi from a sense amplifier 50, a selection gate SGD (SG2) on the bit line side is driven to the voltage VDD, and the program data is inputted to the selected NAND-type cell.
If a program voltage VPGM and a program intermediate voltage VPASS are transferred from the voltage supply circuit CG drivers 6 to their respective word lines CG0 to CG15 at a time t1, a program operation is executed by the program data sent from the bit lines BL0 to BLi. In this case, the program intermediate voltage VPASS (10 V in FIG. 13) is used not only to turn on memory cells (non-selected cells) in the non-selected word lines between the selection gate and the selected memory cell and transfer the program data from the bit line to the selected memory cell but also to generate a non-program voltage in channels in the NAND cell so as not to bring about a threshold voltage shift to the selected memory cell.
Incidentally, in a circuit configuration shown in FIG. 11, when the supply of the voltage VPGM starts at the time t1, the level of the voltage VPGMH for transferring the program voltage VPGM by transfer transistors 3 is lowered by parasitic capacitance of a path for transferring the voltage VPGM and word line capacitance. As the capability of the VPP pump circuit 30 is lower and the load of the VPGM transfer path is larger, this tendency becomes stronger. On this occasion, the voltage VPGM is generated so as to become a voltage lower than the voltage VPGMH by a threshold voltage of an NMOS transistor 25 at an output part of the VPP PUMP 30, and hence it operates together while having at least a difference Vtn corresponding to the threshold voltage of the NMOS transistor 25. Accordingly, when a voltage with the same voltage as the voltage VPGMH is transferred to gates Transfer G of the transfer transistors 3 in the selected row decoder, as shown by (A) in FIG. 13, a voltage of 20 V+Vtn is transferred to the gates Transfer G of the transfer transistors 3, whereby 20 V can be transferred to the selected word line.
Next, FIG. 14 shows the circuit configuration of a level shifter 2 in the block row decoder 10. A D-type NMOS transistor 90 is provided, and the control signal VRDEC of 20 V+α is inputted to a source of the transistor 90. The gates Transfer G of the transfer transistors 3 are connected to a gate of this NMOS transistor 90. A PMOS transistor 91 is connected to a drain of the D-type NMOS transistor 90, and an output 2 of a decoder is connected to a gate of the transistor 91. An NMOS transistor 92 is connected to a drain of the PMOS transistor 91, and the voltage VDD is inputted to a gate of the transistor 92. An output 1 of the decoder is inputted to a source of the NMOS transistor 92.
When the block decoder including this level shifter is selected, the output 1 of the decoder becomes the voltage VDD, and the output 2 of the decoder becomes 0 V. Therefore, the NMOS transistor 92 is cut off after transferring a voltage VDD−Vtn to the gate Transfer G, the D-type NMOS transistor 90 transfers a voltage corresponding to the gate Transfer G to a well and a source of the PMOS transistor 91, and then the PMOS transistor 91 is turned on. Consequently, the voltage transferred by the NMOS D-type transistor 90 is transferred to the gate Transfer G via the PMOS transistor 91 turned on. By this voltage, the NMOS D-type transistor 90 transfers a higher voltage. As stated above, positive feedback is given between Transfer G and the transistors 90 and 91, whereby the voltage to be applied to VRDEC is applied as shown by an arrow in FIG. 14.
In the aforementioned related semiconductor memory device, the following problems arise. Due to a problem in terms of an operation margin, a case where the voltage of VPGMH cannot be fully transferred to the gates Transfer G of the transfer transistors 3 as shown in a waveform C1 in FIG. 13 may occur.
The relation between the threshold voltage of the NMOS D-type transistor 90 and current amount in the level shifter 2 is shown now in FIG. 15. As the voltage is applied to its source, the threshold voltage becomes larger by back gate bias effect. When back gate bias characteristics greatly deteriorate for some reason as in a case 2 although characteristics of a case 1 is assumed at the time of the application of a back gate bias of 20 V+Vtn at the beginning of a circuit design, that is, it is assumed that the NMOS D-type transistor is in an ON state until it transfers VPGMH, the voltage shown in FIG. 14 which the level shifter can transfer lowers greatly.
On this occasion, in terms of a DC operation, after the level shifter is charged to a voltage such as shown by a waveform C1 shown by a broken line in FIG. 13, the gates Transfer G of the transfer transistors 3 become floating. When the voltage of the voltage VPGMH to which the control signal VRDEC is charged lowers as stated above since the program voltage VPGM (20 V) is transferred to the word line at the time t1, the voltage of the gates Transfer G of the transfer transistors 3 lowers concurrently therewith, and the gates Transfer G of the transfer transistors 3 are biased in terms of DC until a time tf. Since the gates Transfer G of the transfer transistors 3 become floating after the time tf, the voltage of the gates Transfer G of the transfer transistors 3 slightly rises by coupling effect even after the time tf by contribution of the program voltage VPGM and the program intermediate voltage PASS in the middle of activation, and a waveform C2 is obtained. The voltage of the waveform C2 is higher than the voltage of the waveform C1, but unless it rises to 20 V+Vtn which is the voltage of VPGMH, the voltage to be transferred to the selected word line becomes a low voltage shown by the waveform C2, and hence the predetermined voltage VPGM (20 V) cannot be transferred. Namely, the program voltage VPGM cannot be completely transferred.
In a normal state, the waveform of voltage of the transfer gates is shown by A, and corresponds to the voltage of the waveform VPGMH. The voltage VPGMH is a voltage obtained by adding the threshold voltage of the transfer gate to the program voltage of 20 V to be applied to the selected word line, and hence the program voltage of 20 V is transferred to the word line without any problem under normal circumstances. More specifically, in the related VRDEC driver circuit 150, by maintaining the control signal VRDEC_V at “H” level from the beginning to the end of a program operation during the program operation and outputting the voltage VPGMH to the gates of the NMOS D-type transistors 152 from the level shifter 151 in the VRDEC driver circuit 150, the VRDEC driver circuit 150 continues outputting the voltage VPGMH. Therefore, if the level of the fundamental voltage VPGMH of the VPP pump circuit 30 lowers when the program voltage VPGM is transferred to the word line, the voltage VPGMH whose level has lowered is also transferred as it is to the control signal VRDEC.
In such a case, the desired program voltage cannot be sometimes applied to the word line of the memory cell under the influence of the problem of the level shifter in terms of the operation margin.